Binary-coded information is widely used in industry today, both for computational purposes and for control of equipment and machinery. Frequently a computer is used to control equipment that may be located remotely from the computer, and it is necessary to provide a communications link between the computer and the equipment. If a computer is controlling many operations or controlling a number of different pieces of equipment, it is frequently necessary to have a data link that will provide high communication rates and good isolation from interference. Fiber-optic links meet these criteria well due to the high bandwidth possible with optical signals and the fact that a fiber-optic cable can be easily isolated from interfering signals.
One of the problems encountered to date, however, in using fiber-optic data links is the need to generate optical signals that can be easily and unambiguously decoded at the other end of a fiber-optic cable. A straight binary signal, i.e. turning a light "on" for a high state and "off" for a low state, may, of course, be used; but such a signal presents several problems in decoding. Since control signals used in process-control applications may have varying rates and may not be synchronous with a clock, it is necessary for the receiver to have full gain at D.C. as well as frequencies up to and including the maximum data rate for an asynchronous control signal that may stay indefinitely in the high or low state. Due to the high gain at D.C. and low frequencies, such a system is susceptible to zero drift, low frequency noise and other forms of interference. In addition, there is no way of knowing whether a low state that is indicated by the absence of light is due to the data transmitted or a break in the optical link.
Numerous codes have been proposed and used in an effort to solve the problems indicated in the foregoing paragraph. One of the best-known codes is known as the Manchester code, which comprises a signal at a constant frequency which is equal to the highest data rate of the binary information to be encoded. The phase of the Manchester-coded signal is either 0.degree. or 180.degree. according to the binary information and the phase of the signal can be detected and decoded to reconstitute a binary signal. While this signal overcomes the problems of low frequency noise interference, since full gain at D.C. is not required of the receiver, it has the problems that it is inefficient in the use of bandwidth and that time quantization is necesary, preventing the transmission of pulse trains with continuously variable frequency. It does, however, provide an indication of whether the link is operative or not. Other codes have also been proposed which have similar problems. They are described, for example, in Proceedings of the IEEE, "Line-Coding Plan for Fiber-Optic Communications Systems" by Y. Takasaki, et al., page 1081 to 1082, July 1975 and IBM Journal of Research Development, "Introduction to Pseudoternary Transmission Codes" by A. Croisier, page 354 to 367, July 1970.